Revised 24 May 2019

The crude birth rate does not compensate for changes in age composition. A better indicator of birth trends is the general fertility rate, the number of births per 1,000 women per year between the ages of 15 and 44. A more meaningful fertility indicator is the average completed family size or total fertility. At the replacement fertility level, each family on average would comprise of 2.11 children. The extra 0.11 child on average compensates for those children who do not survive to reproduction age and for those women who do not have children by virtue of choice or infertility. 

Figure 9 shows the total fertility for the world population from 1960 to 2016. Once again, the annual rate of change in total fertility is illuminating. 

Figure 9: World Total Fertility 1960 to 2016 (Data: United Nations 2019)

World total fertility increased to a maximum of 5.07 children per family in 1964 and then declined monotonically to 2.44 children per family in 2016. The rate of decline in total fertility increased to a level of ‑3.03% per year in 1975, slowed down to a level of -0.82% per year in 1985, increased again to a level of -2.84% per year in 1993, and then slowed down again to a level of -0.39% per year in 2016. Apart from some fluctuations, the largest improvements in reductions in total fertility occurred between 1964 and 1975 and then between 1985 and 1993. 

The slowdown in the rate of decline in total fertility since 1993 is not good news. The world population was 7.7 billion people in January 2019 (United Nations, 2019). If the world population were to continue to grow exponentially at the same annual growth rate of 1.07% from 2018 to 2019, then the world population would double in size within 66 years to 15.4 billion people in the year 2085. 

Table 3 compares the size, proportion, and increase in the world population by income. 

Table 3: World Population by United Nations Income Classification (Data: United Nations, 2019)



1960 to 2017

Income Group


% of World Population


% of World Population

Increase in Size

high-income Countries





1.60 X

middle-income Countries





2.66 X

low-income Countries





4.39 X






2.48 X

In 1960 the high-income countries formed 25.7% of the world population. By 2017 these countries formed 16.6% of the world population due to the low annual growth rate of this income group. The middle-income countries formed 68.8% of the world population in 1960 and by 2017 formed a slightly larger proportion of 73.7%. The low-income countries formed only 5.5% of the world population in 1960 and almost doubled that proportion to 9.7% by 2017 due to a rapid annual growth rate. 

Between 1960 and 2017 the high-income, middle-income, and low-income countries have respectively added 469 million, 3,464 million, and 566 million people to the world population. Emigration from one income group of countries to another conceals the true extent of the above statistics. Total fertility in each income group of countries better signals the source of population growth. 

Figure 10 compares the total fertility and its annual rate of change from 1960 to 2015 for the above income groups. 

Figure 10: World Total Fertility by Income 1960 to 2015 (Data: United Nations 2019)

Total fertility tends to drop with prosperity. There are many reasons why, including low Infant mortality due to better medical care thus less reason to over reproduce as insurance towards being taken care of in old age, better nutrition, and access to medicine to combat life-threatening disease. The trend for nuclear families in high-income countries with both parents working during child rearing years also encourages smaller families. Total fertility also drops with education on family planning and access to contraceptives.  Global statistics on the prevalence of contraception are limited. Table 4 shows the 

United Nations Population Division statistics on the percentage of women who are practicing, or whose sexual partners are practicing, any form of contraception. The statistics are usually measured for women ages 15-49 who are married or in union.

Table 4: Contraception Prevalence in High-income, Middle-income, & Low-income Countries

 (Data: United Nations 2019)


Contraception Prevalence (%)

















Use of contraception in the low-income countries has increased substantially from 1990 to 2014 (16.8% to 34.2%), but by 2014 was almost half of that in the high-income and middle-income countries. The reasons why include cultural lack of emancipation within some countries, lack of education on family planning, and lack of access to contraceptives.  

Total fertility in the high-income countries averaged 3.04 children per family in 1960 and declined to 1.68 children per family by 2015. Total fertility in the high-income countries has been below replacement level of 2.11 children per family since 1976. Population growth, if any, in high-income countries since 1971 would be due to population momentum and positive net migration. 

The rate of decline in total fertility in the high-income countries is of no great concern with regards to the prospects of achieving global ZPG, even though there have been spikes of increases in total fertility. The natural populations of these countries, excluding migration, have already achieved the primary requirement of ZPG that total births within each country should not exceed total deaths. There is a caution here though. Consistent crude birth rates well below that of crude death rates can result in potential undesirable changes in the age dependency ratio which is addressed in more detail below. 

Total fertility in the middle-income countries averaged 5.62 children in 1960 and declined to 2.33 children per family by 2015. The rate of decline in total fertility of the middle-income countries closely parallels that of the world population. As early as 1960 this group of countries already formed the largest proportion (68.8%) of the world population thus dominating the slowdown in the decline of the world total fertility since 1993. The rate of decline in total mortality in the middle-income countries slowed down to a rate of change of -0.46% by 2016. 

Total fertility in the low-income countries averaged 6.57 children per family in 1960 and declined to 4.63 children per family by 2015. Although this change towards a lower total fertility has been less dramatic than that of the high-income and middle-income countries, the rate of decline has consistently improved to a level of -1.54% per year by 2015, a rate of change that exceeds that of the middle-income countries in 2015. 

In 2017 the world population was 7.53 billion and the populations of the high-income, middle-income and low-income countries were 1.25 billion, 5.55 billion, and 0.73 billion respectively (United Nations, 2019). The population of the middle-income countries was 7.60 time larger than that of the low-income countries. Consider the scenario where there is no migration between different income groups of countries and the natural populations of the high-income, middle-income, and low-income countries continue to grow unabated at their respective 2017 annual rates 0.56%, 1.11% and 2.58% (United Nations, 2019). 

Figure 11a shows the populations of the middle-income and low-income countries continuing to grow in size until they are the same size of 26 billion during the year 2157. Figure 11b shows growth in the additions to the world population of 7.53 billion people since 2017 contributed by the middle-income and low-income countries. 

Figure 11a: Growth in Population (Data: United Nations 2019)

Figure 11b: Growth in Population Additions

(Data: United Nations 2019

Additions of population since 2017 by the middle-income countries exceed that of the low-income countries until the year 2140 during which year the additions of populations in both income group of countries total 16 billion people. The population of the high-income countries would increase from 1.25 billion to 2.48 billion people by 2140, an increase of 1.23 billion. In 2140 the world population would approach 41 billion people.  

Irrespective of whether the above scenario is realistic or not, the scenario demonstrates that continued growth in the population of the middle-income countries will dominate future growth in the world population before the year 2100.